Preheat oven to 350 F. Line one 12-cup and one 6-cup muffin pan with paper liners.

Melt chocolate and butter together in top of large double boiler or in large bowl in microwave using the melt chocolate setting; stir until smooth and combined, then cool to lukewarm.

Whisk in brown sugar until well blended, then eggs one at a time, whisking until completely absorbed before adding the next. Whisk in vanilla extract and salt.

Toss the nuts and chocolate chips in the flour, add to batter and fold until completely combined.

Lightly spray the cupcake liners with non-stick spray and use an ice cream scoop to fill dividing batter evenly among each (about 3/4 of the way full). Bake about 25 minutes or until a toothpick in center shows some moist crumbs, and the tops are dry and crackly.

Beat peanut butter and butter together on medium speed until creamy and well combined. Beat in vanilla.

Add confectioners sugar a cup at a time alternating with a tablespoon of heavy cream until it reaches a spreadable consistency (not too thin or thick, when you lift the beater it shouldn't drip right off). Beat 3-4 minutes longer on medium speed until light and fluffy. Frost cupcakes immediately.